1. Field of the Invention
The present invention relates to a packet transfer method and apparatus, and in particular to a packet transfer method and apparatus in a relay line between routers.
2. Description of the Related Art
Services of a network carrier using an ATM (Asynchronous Transfer Mode) technology include the following two services:    (1) CBR (Constant Bit Rate) type: Rate guaranteed service guaranteeing a fixed bandwidth;    (2) GFR (Guaranteed Frame Rate) type: Partial rate guaranteed service (or rate semi-guaranteed service) having both of a best-effort part and a guaranteed part (guaranteed rate part).
In the above-mentioned rate guaranteed service (1), a fixed rate is constantly guaranteed. Namely, the rate guaranteed service is used for communication of an application constantly requiring a fixed rate bandwidth, and communication of an application which is weak in a delay fluctuation or a data loss. Packets exceeding the maximum rate are discarded.
In the above-mentioned partial rate guaranteed service (2), the minimum (contract) guaranteed rate is constantly guaranteed in a Mega Data Netz (MDN) line or the like. In the same way as the above-mentioned rate guaranteed service (1), cells exceeding the maximum rate are discarded. Since the bandwidths equal to or more than the minimum guaranteed rate and equal to or less than the maximum rate are commonly used by a plurality of users, and may be used with the maximum rate as an upper limit depending on a traffic situation, this partial rate guaranteed service is suitable for a TCP/IP communication or the like performing a flow control and retransmission processing.
In such a partial rate guaranteed service, as exemplified in FIG. 8, a virtual path (VP) bandwidth shown by thick lines is established as a relay line 5 between routers (stations) 3 and 4.
Supposing that e.g. virtual channel (VC) bandwidths for respective users (terminals) A, B, and C are assigned for the virtual path bandwidth of the relay line 5, the virtual channel bandwidth for the user C is guaranteed with equal to or less than a maximum rate Rmax and equal to or more than a minimum guaranteed rate Rg (rate of 10%, 50%, or the like of the maximum rate) as shown in FIG. 8. The same applies to the other users A and B. Accordingly, the virtual path bandwidth of the relay line 5 is set smaller than a total value of the virtual channel guaranteed bandwidths for the users.
When such a partial rate guaranteed service is applied to the relay line between routers, the routers can not recognize the guaranteed bandwidth (guaranteed rate) of the line in real time. For this reason, the router on the transmission side constantly is to transmit through a relay line, data forwarded (transferred) at the maximum rate Rmax like the virtual channel bandwidth for the user C as shown in FIG. 8. Otherwise, effective use of the line service can not be maximized if the transmission at the maximum rate is not performed although it is actually enabled.
However, since the virtual path bandwidth of the relay line 5 is set smaller than the total value of the virtual channel guaranteed bandwidths for the users as mentioned above, when the virtual channel bandwidth for the line of the user C is reduced to the maximum rate or less by a competition with the user A or the user B, the router 3 continues the data transmission at the maximum rate as heretofore. The part exceeding the actual VP bandwidth of the line at that time is buffered somewhere in the network. However, if the part exceeds the buffering limit, the part is discarded in the network.
In this case, the amount of data which is actually buffered and kept within the network without being discarded is not published by the network carrier side under the present circumstances.
Accordingly, when a data discard occurs in the relay line 5 between the routers 3 and 4, and e.g. a communication application (software) between end users uses TCP protocol, the TCP protocol detects the data discard, performs the retransmission control of the data between the end users, and performs a slow start control of the transmission rate between the end users, thereby extremely reducing the communication rate.
Subsequently, the transmission rate is gradually increased by the flow control of the TCP application between the end users. When the packet discard occurs again in the line part, a slow start, a transmission rate increase, and a packet discard are repeated.
As a result, there is a problem that retransmission packets are increased within the network and the slow start control of the communication between the end users due to the packet discard results in the reduction of the communication efficiency.
On the other hand, there is a QoS provision method setting a traffic category (one of CBR, VBR-rt, VBR-nrt, UBR, and GFR) in an IP header when a user side transmits an IP packet in order to guarantee a quality of service by separating traffic categories into a best effort type category and a real time type category, selecting either of the traffic categories mentioned above based on the IP header when each router receives the IP packet, and guaranteeing the quality of service of the concerned traffic category (see e.g. patent document 1).
Furthermore, there is a bandwidth securing VPN configuring method establishing an IP tunnel between routers connected to the Internet in order to secure a bandwidth of a VPN per cost or subnet, reserving a transmission bandwidth of the IP tunnel by starting up a network resource reservation protocol on the IP tunnel, thereby securing the bandwidth of the VPN per cost or subnet (see e.g. patent document 2).
<Patent Document 1>
Japanese Patent Application Laid-open No. 2001-189754 (FIG. 1 & Abstract)<
Patent Document 2>
Japanese Patent Application Laid-open No. 10-70566 (FIG. 1 & Abstract)